1. Field of the Invention
This invention relates to a focusing circuit for cathode ray tube (CRT) display which automatically adjusts focusing on an optimized point on a screen of a television receiver set in accordance with brightness and darkness of a picture.
2. Description of Prior Art
FIG. 1 is a circuit diagram showing a prior art focusing circuit for a television receiver set. The focusing circuit comprises a horizontal output transistor 1, a damper diode 2, a resonance capacitor 3, a deflection coil 4, an S distortion correction capacitor 5, a flyback transformer (FBT) 6, an anode voltage (hereinafter referred to as E.sub.HV) rectifying diode 7, a focusing voltage (hereinafter referred to as E.sub.F) rectifying diode 8, an E.sub.F capacitor 9, fixed resistors 10 and 12 for determining a range of change of the E.sub.F, a variable resistor 11 for adjustment of the E.sub.F, a first output terminal 30 for the E.sub.HV, a second output terminal 31 for the E.sub.F, and a third output terminal 32 connected to a focusing electrode (not shown). The E.sub.F delivered out via the variable resistor 11 is supplied to the focusing electrode of the CRT (picture tube) so as to determine focusing on the screen.
Usually, a focus tracking characteristic of the picture tube (represented by a beam current value in abscissa and a ratio E.sub.F /E.sub.HV in ordinate) is determined by a characteristic of the FBT 6. In many applications, the focus tracking characteristic of the FBT 6 is so set that the relation between the beam current of the CRT and the ratio E.sub.F /E.sub.HV is always substantially constant as shown in FIG. 2. However, an optimum focus tracking characteristic for a CRT serving as a picture tube of a television receiver set is such that the ratio E.sub.F /E.sub.HV changes with the beam current and therefore, the aforementioned characteristic wherein the ratio E.sub.F /E.sub.HV is substantially constant relative to the beam current disadvantageously degrades focusing in accordance with changes in brightness and darkness of a picture.
In addition, when a video signal being displayed on the screen contains a white peak signal component representative of a white character, a delay is caused between the E.sub.F and the E.sub.HV on account of the capacitor 9, giving rise to defocusing on the screen.